The present invention relates to thin film electroluminescent displays.
The configuration of the thin film electroluminescent display (TFEL) to which the invention is directed is generally shown in FIG. 1. An electroluminescent phosphor is sandwiched between two dielectric layers. A transparent conductor and a back electrode are used to selectively address the individual pixels of the display.
A pre-eminent difficulty in the commercial application of thin film electroluminescent displays has been the high voltages which are typically required. Since electroluminescence is produced in the phosphor 10 only at an electric field strength of about a million volts per centimeter or more, the drive voltages which must be applied to the conductors and are quite high.
One of the factors which exacerbates the high voltage requirement is the fact that the electroluminescent phosphor must be made thick enough to be sufficiently luminous to provide an adequate signal-to-noise ratio in the display. That is, a TFEL display which was constructed with a very thin phosphor layer might permit lower drive voltages, but would also be so dim in its "on" condition that the device would not, in practice, be useful as a marketable display.
Typically, the drive voltages required in a TFEL display according to the prior art will be substantially higher than those used in plasma display panels, and may be as high as 200 volts or more. Such high display address voltages mean that the display driver circuits required are very expensive and are physically large. Thus, the majority of the cost of a TFEL display formed according to the present art is attributable to the cost of the very high voltage drivers required.
A further constraint on the design of TFEL displays is heat dissipation. Heat will normally be dissipated both in the phosphor and in the dielectric layers during operation of the device. If the heat dissipated is excessive, the phosphor temperature may rise to a level at which avalanche multiplication of carriers will take place under the very high electric fields applied, and catastrophic break-down promptly follows.
Thus, it is an object of the present invention to provide a thin film electroluminescent display incorporating a phosphor which provides a large luminance for a given electric field magnitude.
It is a further object of the present invention to provide a thin film electroluminescent display incorporating a phospor which has minimal heat dissipation for a given electric field magnitude.
It is a further object of the present invention to provide a thin film electroluminescent display incorporating a phosphor which has high luminance and low heat dissipation.
It is a further object of the present invention to provide a method for fabricating thin film electroluminescent displays in which the phosphor has high luminance and low heat generation.
Thermal annealing of thin-film electroluminescent phosphors has been used to enhance crystallinity. However, temperatures much above 500 degrees C. are impractical, since the normal substrate materials begin to soften.
Thus, it is a further object of the present invention to provide good quality annealing of electroluminescent phosphors without risking substrate softening.